It is known that a hydrogenated aromatic vinyl compound polymer that is obtained by hydrogenating the aromatic ring of an aromatic vinyl compound polymer, a hydrogenated block copolymer that is obtained by hydrogenating the double bonds (that are derived from an aromatic ring and a diene) of a block copolymer that includes a polymer block that includes a repeating unit derived from an aromatic vinyl compound as the main component, and a polymer block that includes a repeating unit derived from a linear conjugated diene compound as the main component, can be used as an optical film (e.g., polarizing film and retardation film), and it is possible to provide the desired retardation to a film formed by extrusion by stretching the film (see Patent Documents 3 to 6).
A film that is formed of a thermoplastic resin and used for a device (e.g., liquid crystal display device) that utilizes polarized light is required to have optical transparency, show only a small change in the in-plane retardation in the planar direction and the thickness direction, and rarely cause a phenomenon in which an image is distorted due to the lens effect of the convexities and concavities of the surface of the film, for example.
Therefore, a highly uniform thickness is required for an optical film, and an optical film has been produced using a solution casting method in order to obtain a film having a uniform thickness.
Since the solution casting method has problems in that the solvent used therefor causes environmental pollution, and it is difficult to achieve high productivity, a melt extrusion method has been used in recent years. However, the melt extrusion method also has problems in that the thickness of the resulting film easily varies, and a die line easily occurs in the film extrusion direction.
Examples of the die line include a die line (linear trace) that is formed when a molten resin that is extruded from the die adheres to the wall surface of the die, a die line that is formed due to a resin that adheres to the opening of the lip of the die, and the like. These die lines consist of ridges and valleys that have a height of about 0.1 to 0.5 μm and a width of about 50 to 500 μm.
When an optical film has such a die line, an optical signal error may occur, or the pattern of the die line may be displayed on the display, for example. Therefore, attempts to suppress a die line have been made by adjusting the temperature of the molten resin, appropriately selecting the melt viscosity, providing an air gap between the cooling roll and the die, or applying a voltage to the molten resin film that comes in contact with the cooling roll when extruding a film, for example. The lip of the die may be subjected to a polishing treatment or a plating treatment (e.g., chromium plating).
However, these methods cannot sufficiently prevent the occurrence of a die line, and a die line may occur during production, whereby various problems may occur in terms of the production, the stability of the product properties, and the like.
Various measures have been proposed in order to solve the above problems.
For example, Patent Document 1 discloses a transparent resin sheet that is formed of a cycloolefin-based thermoplastic resin, at least one side of the transparent resin sheet forms a flat and smooth surface having a surface roughness of 0.01 μm or less, and the transparent resin sheet having a thickness of 0.05 to 3 mm and a residual retardation of 20 nm or less. Patent Document 1 also discloses a method that produces the transparent resin sheet by causing the cycloolefin-based thermoplastic resin (in a molten state) that has been extruded from a T-die that is provided to an extruder to adhere to a metal cooling roll or a metal cooling belt under pressure by compressing the cycloolefin-based thermoplastic resin between the cooling roll and the cooling belt, and removing the cycloolefin-based thermoplastic resin from the cooling roll or the cooling belt at a temperature equal to or lower than the glass transition temperature of the cycloolefin-based thermoplastic resin.
Patent Document 2 discloses a method for producing a cycloolefin resin extruded product (sheet or film) that includes melting a cycloolefin resin, extruding the melted cycloolefin resin through a die that has a lip having a peel strength of 75 N or less to form the cycloolefin resin. Patent Document 2 states that the resulting cycloolefin resin extruded product has excellent surface flatness and smoothness, and can suitably be used for optical applications.
Patent Document 3 discloses a method for producing an optical film that includes melting an amorphous thermoplastic resin having an alicyclic structure using an extruder, extruding the melted amorphous thermoplastic resin in the shape of a sheet from a die, forming the extruded sheet-shaped amorphous thermoplastic resin by bringing the extruded sheet-shaped amorphous thermoplastic resin into contact with at least one cooling drum, and taking up the formed hydrogenated block copolymer [2], the die being designed so that the average value of the surface roughness Ra of the die lip is 0.05 μm or less, and the distribution range of the surface roughness Ra over the width of the die lip is ±0.025 μm or less with respect to the average value to obtain an optical film characterized in that the height from the peak of a ridge of a die line to the bottom point of a valley that is contiguous to the ridge is 100 nm or less over the entire surface of the optical film, and the slope of the die line is 300 nm/mm or less over the entire surface of the optical film.
Patent Documents 4 and 6 disclose that an optical compensation film and an optical film having small retardation can be obtained by utilizing a hydrogenated block copolymer in which both a polymer block derived from an aromatic vinyl compound and a polymer block derived from a conjugated diene compound are hydrogenated. Such a hydrogenated block copolymer can be supplied on a large scale as compared with a thermoplastic resin that is synthesized from a cycloolefin and has an alicyclic structure since the aromatic vinyl compound and the conjugated diene used as the raw materials are readily available, and it has been desired to put an optical film that utilizes such a hydrogenated block copolymer into practical use.